Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems, (e.g., a Long Term Evolution (LTE) system). A wireless multiple-access communications system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, which may be otherwise known as user equipment (UE).
CDMA, TDMA, FDMA, and OFDMA systems may communicate with multiple UEs through the use of resource sharing and/or orthogonal transmissions. In some cases, separate communications to multiple UEs may be accomplished by strategically sharing resources or by orthogonally transmitting to the UEs over simultaneously-shared (“common”) resources. For instance, a TDMA system may designate time intervals for transmissions during which a UE is scheduled to receive a transmission—e.g., the base station may transmit to a first UE in a first time interval, a second UE in a second time interval, etc. An FDMA system may simultaneously communicate with multiple UEs by sending UE-specific transmissions over corresponding frequency resources allocated to each of the UEs. The FDMA resources may include subcarriers that are separated in frequency in such a way that transmissions over one subcarrier are orthogonal with transmissions over another subcarrier. And OFDMA may utilize a combination of TDMA and FDMA techniques. CDMA systems may simultaneously transmit to each of the UEs using the same time and frequency resources, but may uniquely modulate transmissions to different UEs with an orthogonal code. The UEs may be assigned unique orthogonal codes, and may apply the orthogonal codes to received signals to identify the transmission intended for that UE.
In some cases a wireless communications system may utilize a non-orthogonal multiple access (NOMA) system that shares time and frequency resources without using orthogonal transmissions to support communications with multiple UEs. For example, a NOMA transmission may include multiple streams of data intended for multiple UEs using common resources—e.g., at least partially overlapping time, frequency, and/or spatial resources—but may transmit the streams of data without uniquely orthogonalizing the transmissions to the different UEs. NOMA transmissions may take advantage of the physical locations of the UEs in the wireless communication system to enhance the overall data throughput of the resources. For instance, the base station may transmit an enhancement layer (EL) to a first UE that has a relatively higher geometry (e.g., higher signal-to-noise ratio (SNR), which is typically associated with a UE that is closer to the base station) using overlapping resources with a base layer (BL) intended for a second UE that has relatively weaker geometry (e.g., lower SNR, which is typically associated with a UE that is farther from the base station). The NOMA transmission layers may be multiplexed in various ways including by using different transmit power levels (e.g., superposition), hierarchical modulation, or other multiplexing techniques.
For multi-layer transmissions, such as NOMA or MIMO transmission, a UE may use interference cancellation techniques while decoding a received signal. For instance, a UE may use interference cancelling techniques to cancel the effects of transmissions to co-scheduled UEs on one transmission layer, prior to decoding a signal received by the UE on another layer. It may be beneficial for a base station to provide, to a first UE, certain information describing aspects of a transmission layer associated with a co-scheduled UE. However, sending control information intended for the second UE directly to the first UE may be duplicative and significantly increase control overhead. Moreover, indicating to the first UE how to decode control information intended for the second UE may inhibit the ability to dynamically assign UEs to different transmission layers. Indicating to the UE information for decoding control information for the second UE may also introduce a significant increase in the amount of control information sent to the first UE and/or processing that the first UE may have to perform to locate control information associated with the other transmission layer, which may significantly impact battery life.